The borderline hypertensive rat (BHR): a new model for the study of environmental factors in the development of hypertension.

While many studies have attempted to produce hypertension through the use of various environmental stressors, few have succeeded in producing chronic elevations in blood pressure beyond levels considered to be borderline hypertensive (140-160 mm Hg systolic). The problem with most studies stems from the use of genetically normotensive animals and the selection of stressors to which animals readily adapt. A new approach is suggested, which recognizes the role of genetics in human essential hypertension. Animals with one hypertensive parent do not develop spontaneous hypertension but show a more sensitive cardiovascular response to environmental stressors than animals with normotensive parents. Preliminary studies revealed that animals with a mixed genetic history of hypertension develop spontaneous borderline hypertension. When subjected to shock-shock conflict, these borderline hypertensive rats (BHR) developed permanent hypertension that failed to abate even after a ten-week, shock-free recovery period. The hypertension was accompanied by elevated heart weight to body weight ratios and by significant cardiac pathology. Subsequent work has demonstrated that these animals also become hypertensive when fed a high-sodium diet. Finally, in a series of exercise studies, we found that BHRs subjected to a shock stressor were protected against stress-induced hypertension if they exercised daily. The potential of this model for studies of the mechanisms by which environmental variables produce permanent hypertension is discussed.